This application claims the priority of German patent Application No. 100 44 228.5 filed Sep. 7, 2000, which is incorporated herein by reference.
The invention relates to a method for stacking containers of thermoplastic material, which are molded from/punched out of a heated foil web in a molding/punching tool and are transferred from this tool, either directly or indirectly, via a transfer device to a stacking magazine or to a catch plate and are removed from there as a container stack. The invention furthermore relates to a device for realizing this method.
Containers that are molded and punched out in a thermal molding machine with a combination molding and punching tool are generally transferred after ejection from the molding/punching tool to stacking magazines. This can occur directly by displacing the molding tool bottom and then transferring the containers to the stacking magazines where they are retained. A method of this type is disclosed in the German patent DE 33 46 628 C2.
A method of transferring containers with a transfer device that grips the ejected containers and, following a rotation of 180xc2x0, inserts these into the stacking magazines is furthermore disclosed in the German patent reference DE 198 52 359 A1. A non-rotating transfer device, meaning for a linear conveyance of the items, which uses a catch plate is known from the German patent reference DE 198 12 414 A1. One container is picked up during each cycle and is transferred to a stacking magazine.
The German patent reference DE 198 48 628 A1 also discloses the use of a catch plate, in which complete container stacks are formed, however, which are then transferred to a stacking magazine. The stacking magazine is then guided to a transfer location for successively conveying all stacks.
The German patent DE-AS 27 14 352 also discloses the stacking of several containers in a catch plate. The container stacks are then gripped and conveyed further with a transfer device consisting of rods.
With all these known methods, the containers forming a stack in a catch plate or a stacking magazine are displaced upon the insertion of the following container by a measure or distance that corresponds to the stack measure, meaning the distance between two neighboring stacked containers. This forward movement of the container stacks is realized in the German patent reference DE 198 12 414 A1 with the gripping elements in the shape of mandrels, which impact with points along the edge of the container that is stacked in last and thus cause the container stack to advance when the new container is transferred to the stacking magazine.
Cycle numbers of up to and exceeding 40 cycles/minute are achieved with modern automatic thermal molding machines, meaning little time is available for stacking the containers and these must consequently be stacked very quickly. In particular, this is true when stacking containers directly into stacking magazines or a locally fixed catch plate. Advancing the existing container stacks by the distance measure must occur during an interval in the order of magnitude of 0.1 seconds, which is necessary for reaching a cycle time of 1.5 seconds with 40 cycles per minute for the following processing steps: molding-cooling down-punching out-opening the tool-ejecting-closing the tool. In addition, the containers must be removed from the mold while still relatively warm, which is possible in principle, depending on the material type and thickness, but carries the danger that the containers are deformed by a mechanical force effect. This force effect occurs because the containers are moved at the bottom through the displaceable mold bottom of the molding/punching tool and hit the stack edge of the previously stacked containers with the container edge. Depending on the number of previously stacked containers and their weight, a force corresponding to the product of weightxc3x97acceleration (K=mxc3x97b) must be generated and must be absorbed by the container edge and the side walls of the container, up to the mold bottom. In particular with thin-walled containers, which permit a high cycle number due to their rapid cooling inside the molding tool, the danger of deformation of the containers is high and increases with the length of the container stack. In practical operations, the deformation danger limits the number of cycles for the device.
If the containers are advanced with the aid of the mandrels on the catch plate that is moved with each cycle (DE 198 12 414 A1), the container wall and the container bottom do not need to absorb a force during the stacking. However, the complete force for advancing a container stack now becomes effective at three or four points along the edge of the previously stacked container. The container has cooled only slightly in these 1.5 seconds, even though a cycle has passed in the meantime, so that with this forward movement of the stack, the danger of deformation at several points along the container edge exists. The driving speed of the catch plate is reduced relative to the mold bottom speed, but is still relatively high. During the available 1.5 seconds, a total distance of approximately 400 mmxe2x80x94back and forthxe2x80x94must be traversed. Even if a slower movement sequence in the end position of the catch plate movement is included in the calculation, a time in the order of magnitude of 0.3 seconds results for the actual stack lift of approximately 8 mm.
As a result of the relatively rapid stacking of containers with the known methods, as well as the associated acceleration of the complete, previously formed container stack, the danger exists that with flat containers, these will drift apart and the complete stack will thus disintegrate.
Thus, it is the object of the invention to design the method such that when transferring containers that are still warm and/or thin-walled to stacking magazines or a locally fixed catch plate, these are not deformed and do not drift apart during the stack formation, despite high cycle numbers.
The above object generally is achieved according to a first aspect of the invention by a method for forming a stack of containers of a thermoplastic material, which are molded from/punched out of a heated foil web in a molding/punching tool, which method comprises: transferring containers consecutively from the tool, either directly or indirectly, via a transfer device to a first position within one of a stacking magazine and a catch plate; using a displacement device that is separate from the transfer device, consecutively displacing each container in an axial direction of the stacking magazine or catch plate from the first position by a distance at least equal to a distance between two adjacent containers of a stack of containers to a second position within the stacking magazine or catch plate to form a stack of containers at the second position, and displace any stack of containers at the second position by at least the distance between two adjacent containers of a stack; and, subsequently axially removing the stack of containers from the stacking magazine or catch plate.
Owing to the fact that the container stacks in the stacking magazines or the locally fixed catch plate are moved with a displacement device, the displacement can occur at a slow speed and thus with little force effect on the last containers that are stacked in. For example, with a cycle number of 40/min, the displacement device has 1.5 seconds to traverse a distance in the order of 2xc3x978 mm (back and forth). Compared to the known method with movable catch plate, a displacement device can take 10 times longer and the displacement can occur at a correspondingly slower speed. The displacement by the stack measure can occur with a separate displacement unit, which only serves this purpose. The stack can be removed with a second device that is known per se, e.g., consisting of rods, once the predetermined number of containers per stack is reached.
However, the container stack can also be displaced by the stack measure with a device for removing the stacks from the stacking magazines or the locally fixed catch plate. The expenditure for this operation is low because it requires only a corresponding drive with varied lift and an adaptation of the control. However, the fact that the time needed for removing the container stacks exceeds the cycle time presents a problem. Thus, two or three containers are stacked in during the removal, without intermediate displacement of the stack measure. For the most part that is non-critical because these few containers have a low weight and no deformation must be feared. The danger only increases with a higher number of containers in the container stack.
This method also makes it possible to displace the container stack inside the stacking magazines or the locally fixed catch plate by a measure exceeding the stack measure without substantially increasing the deformation danger. As a result, it is possible to have a distance between the last container stacked in and the container stack, which is in the order of magnitude of two times to three times the stack dimension. This free-standing feature favors the cooling down of the last-stacked container on the inside surface and the last container of the stack on the outside surface, which can be improved by blowingxe2x80x94preferably cooled and/or sterilizedxe2x80x94air onto both surfaces via jets. It has turned out that the containers are still relatively warm with this high cycle number andxe2x80x94if stacked one inside the otherxe2x80x94don""t cool down well. The above-mentioned cooling step clearly improves the cooling.
The above object generally is achieved according to a second aspect of the invention by an apparatus that is connected downstream of a molding machine and comprises stacking magazines or a catch plate, into which the containers can be stacked directly or indirectly from the molding/punching tool or via a transfer device, and a stack displacement device for displacing all container stacks, which displacement device is connected to a drive and is effective with each cycle of the molding machine, said device being arranged in the region of the intake opening of the stacking magazine or the catch plate.
The method is described in further detail with schematic drawings of the device.